Try to get one that supports sli, I think the 8000's do (the 9000's support three :D) This allows you to upgrade your computer much easier.
That's not really what SLI is meant for and it certainly isn't the answer to anyone's problems in this threads (except maybe myself and Otis).
As others have said, you need a new motherboard, but that will likely mean new RAM, a new power supply and probably a new case. You would need to save/earn up some more money...you might be able to get a nice computer for $400 if you build it yourself.